Natsumi Koshigaya, nicknamed Nattsun or Nacchan, is a first-year middle schooler at Asahigaoka Branch School. The youngest Koshigaya sibling, she embodies a carefree, rebellious spirit, readily abandoning homework for games or pranks. Her mischievous antics often involve deflecting blame onto objects or peers, showcasing a playful knack for avoiding consequences. Standing 155 cm tall, she favors practical, rural-inspired clothing. Her dynamic with older sister Komari centers on lighthearted teasing, particularly about Komari’s height, highlighting their contrasting frames. Beneath their sibling rivalry lies undeniable affection. Though academically disinterested, Natsumi displays sharp intuition for natural patterns and rural tasks, underscoring her practical, nature-savvy intelligence. She shares a bond with Renge Miyauchi, the group’s youngest, marked by mutual respect and adaptability. Natsumi humors Renge’s quirks, adopting her nicknames and patiently fielding her questions, balancing mischief with thoughtful camaraderie. A childhood phase of vowing to marry her brother Suguru—now a cringe-inducing memory—hints at youthful innocence shaping her familial ties. Her mother Yukiko frets over her academic neglect and chaotic tendencies, mirroring clashing priorities. Natsumi adores ramen but scowls at radish sprouts. Restless fidgeting and a habit of casually perches barefoot, even outdoors, amplify her lively, unfiltered charm. Throughout all stories, she embodies a spirited fusion of troublemaking, ingenuity, and steadfast loyalty to loved ones, her core traits unwavering across every season and adaptation.